Fionn Whitehead had to step away. Following an hours-long shoot of the World War II film Dunkirk, in which he plays fresh-faced private Tommy, “I had to go have food with friends and have a laugh,” says the Brit. “I had to shake off the day. Otherwise, I would have been dragged down.”

After all, the Christopher Nolan thriller retells the horrific 1940 battle in Dunkirk, France, where 400,000 Allied soldiers were trapped between the English Channel and the German army. “It was a very intense atmosphere,” he adds. “We shot in Dunkirk. The destruction was sobering.”

Fionn Whitehead as Tommy in ‘Dunkirk’Melinda Sue Gordon/Warner Bros.

The 20-year-old, who went through three months of auditions before landing the role, gets into action for Us.

Us Weekly: While training, was there ever a moment where you thought you couldn’t pull off a stunt?

Fionn Whitehead: You make yourself do the physical aspects for a Christopher Nolan film! There was no excuse not to get into shape. I was surrounded by five burly stunt guys constantly telling me I could do it and forcing me through these exercises. I was extremely scrawny when I auditioned. I jogged on the beach, I swam in the sea in battle gear, which weighs about eight pounds, and I carried stretchers with a 60-pound dummy on top. I thought it was amazing.

Fionn Whitehead attends the ‘Dunkirk' US premiere in New York City.Angela Weiss/AFP/Getty Images

Us: Did filming ever feel too real?

FW: There was not a lot of imagination needed because of the world Chris built and the incredible sets constructed. They did as much as possible to get as close to the real thing. But to say I’ve been through the same thing as those guys would be disrespectful. There was a food truck on set!

FW: This is an event that we should always look back on as a point of reference. It definitely shaped the war. The spirit of Dunkirk is something everyone grew up learning about. After Dunkirk, everyone came together and it set the tone in the U.K.

Dunkirk, which also stars Harry Styles, Mark Rylance and Tom Hardy is in theaters now.